Air flow meters of the thermal type, capable of directly measuring the mass flow rate, have become the mainstream of air flow meters for detecting the intake air flow of an internal combustion engine of an automobile or the like.
The thermal type air flow meters include those of the semiconductor type, comprising a sensor element including a heating resistor and a thermosensing resistor formed in a thin film part (some microns thick) of a semiconductor substrate (e.g., Si) formed by removing part of the semiconductor substrate. Such a thermal type flowmeter of the semiconductor type is installed in an air intake pipeline of an internal combustion engine and used for measuring a quantity of a fluid such as the flow rate of the intake air.
Although dust in the air is captured by an air cleaner arranged on the upstream side, intake air containing foreign substances, such as fine particles that cannot be captured by the air cleaner and carbon and oil diffusing from the combustion chamber's side of the internal combustion engine, flows into the air intake pipeline of the internal combustion engine. Therefore, protection of the sensor element from such foreign substances is necessary for achieving high-accuracy measurement of the intake air flow.
Conventional techniques for protecting the sensor element from the foreign substances contained in fluid include those described in Patent Documents 1 and 2. In the technique described in the Patent Document 1, intrusion of dust and collision of the dust with the sensor element are prevented by arranging an obstacle on the upstream side of the flow rate measurement device or the sensor element. In the technique described in the Patent Document 2, adhesion of liquid substances such as waterdrops to the sensor element is prevented by forming grooves and/or projections on the inner surface of a secondary channel and making the grooves, etc. capture the liquid substances.